The present invention relates to a temperature sensing circuit which produces a digital signal for indicating whether a sensed temperature is above or below a set point.
In many cooling systems such as refrigerators, a temperature sensing device is located in heat conduction relationship with a discharge air duct or the discharge air stream in order to sense discharge air temperature. In some cooling systems, the temperature sensing device is a thermistor which is typically a negative temperature coefficient device, i.e., its resistance increases as the sensed temperature decreases. Because the thermistor develops a low level analog signal, the temperature sensing circuit is particularly susceptible to noise developed in other parts of the cooling system. These noise signals can be caused by the opening and closing of contacts and relays or by noise that is present in the power supply. Additionally, the analog signal can be affected by variations in power supply voltage. Typically, these types of noise are reduced by employing large filter components which are relatively expensive.
Other temperature control systems employ the use of hysteresis to control the duty cycle of the apparatus. The duty cycle can be controlled by adjusting the amount of hysteresis through the use of a potentiometer. These systems are, however, relatively susceptible to noise, which can cause erroneous switching of the refrigerator components.